Acrylic pressure sensitive adhesives (PSAs) have emerged as the product of choice in a variety of end-use applications where color, clarity, permanency, weatherability, versatility of adhesion, or the chemical characteristics of an all acrylic polymer is required. These applications include a variety of consumer, packaging, industrial and health care tapes, paper and film labels, decals, fleet markings/bumper stickers, and the like.
Acrylic PSAs have traditionally been prepared in and subsequently coated from an assortment of hydrocarbon solvents. Aqueous emulsion acrylics have been recognized in recent years as an environmentally safe alternative to solvent-born systems.
Product evolution of acrylic PSA technology has been toward higher solids and a 100% solids hot melt system. Elimination of the carrier has several economic benefits to the adhesive converter including lower energy and conversion costs, increased line speeds and production rates, as well as eliminating any adverse effects of organic solvents.
Given the desirable characteristics of hot melt technology, there has been ongoing interest in developing acrylic hot melt PSAs having the requisite four fold balance of adhesion, cohesion, strechiness and elasticity. The desire to maintain this balance of properties makes it extremely difficult to improve cohesive or internal strength without compromising processability or even the overall pressure sensitivity of the entire system. With acrylic adhesives, long term color stability, weatherability and durability are also important product characteristics.
Early attempts to produce acrylic hot melt pressure sensitive adhesives involved blends of polyacrylates and polymethacrylates having carefully selected compatibility characteristics. These initial blends were found to have a limited service temperature and deficiencies in cohesive strength. Bartman, in U.S. Pat. No. 4,360,638 and U.S Pat. No. 4,423,182 discloses an ionomeric acrylic hot melt PSA system comprising a polymer containing carboxylic acid, a miscible metal salt, and an o-methoxy-substituted aryl acid. While significant improvements in cohesive properties were reported for the acrylic ionomers two problems existed in controlling the ionomeric interactions at processing temperatures, resulting in unstable melt viscosities and limited commercial utility.
Until recently, prior art relating to acrylic graft or comb polymers was restricted to non-pressure sensitive adhesive end-uses. The preparation of styrenic based macromonomers and their copolymerization with acrylates is described by Milkovich et al. in U.S. Pat. No. 3,786,116. That patent teaches the use of this technology for acrylic thermoplastic rubber applications and not acrylic pressure sensitive adhesives requiring the four fold balance of properties. Schlademan, U.S. Pat. No. 4,551,388, Husman et al, U.S. Pat. No. 4.554,324 and European Patent Application Serial No. 104,046 discuss the use of macromonomers with acrylic comonomers in acrylic hot melt pressure sensitive adhesive compositions. While the Schlademen patent focused on the use of styrenic macromonomers, the Husman patent extended the concept to include acrylic PSA compositions based on poly (methyl methacrylate) macromonomers. Schlademan, in U.S. Pat. No. 4,656,213, disclosed that compounding acrylic graft copolymers improves properties but indicated it was essential to have styrenic pendant macromonomer side chains. In addition, only partially hydrogenated rosin ester tackifiers having yellow color were illustrated which would compromise water white color, weatherability and durability features normally associated with high performance acrylic pressure sensitive adhesives.
Finally, since styrenic polymers are known to undergo UV degradation, the macromonomer side chain may also contribute to a deterioration of properties in applications requiring long term outdoor exposure.
Sunagawa et al in Kokai Pat. No. 56[1981]-59882 discloses the preparation and use of acrylic graft copolymers as pressure sensitive adhesives whereas the graft copolymer is synthesized by reacting functional groups along a low Tg main acrylic copolymer with functional groups along a modifying copolymer of higher Tg. However the resulting acrylic graft copolymers were not identified as hot melt candidates nor was it disclosed that compounding improved the balance of pressure sensitive adhesive properties.